Question

(a.) Describe the protection incorporated in AC. electrical switchboards.

(b) State with reasons what maintenance is required for switchboards and associated equipment.

Metal-clad, dead front switchboards are mandatory for ac systems.

Mechanical strength and non flammable construction are obtained with the use of sheet steel for the cubicles

A passage way of at least 0.6m is left at the rear for access

Incorporation of ACB which is set to disconnect from the main bus in case of faults, thus avoiding damage to MCR.

For HV system it should be separately earthed again to prevent internal damages, due to high earth fault current.

Various interlocks are provided, example until the circuit is dead the door cannot be opened.

Protection devices are incorporated

Overload relay / over current protection.

The alternator breaker has an overcurrent trip, but a major consideration is that the supply of power to the switchboard must be maintained if possible.

The breaker therefore is arranged to be tripped instantly only in the event of high overcurrent such as that associated with short circuit.

When overcurrent is not so high, a delay with inverse time characteristic allows an interval before the breaker is opened.

During this time the overload may be cleared.

Reverse power protection

Alternators intended for parallel operation are required to have a protective device which will release the breaker and prevent motoring if a reversal of power occurs.

Under voltage protection.

Closure by mistake of an alternator breaker when the machine is dead is prevented by an under-voltage trip.

High low frequency trip.

The relay consist of electronic circuitory. Once a high low frequency is detected by transducer [61.5/58.5] the breaker is tripped.

Maintenance.

Indicator lamp test to be carried out at regular intervals to ensure all in working order. Replace faulty lamps.

Visual examination of circuit breaker in main and emergency and distribution boards.

Check condition of main arcing, auxiliary contacts for signs of wear, misalignment and overheating. If sevewr burning or pitting occurs to the main contact they need to be filed and alignment of contact checked.

The arcing contact are normally subjected to burning and can be dressed with smooth file. Emery paper should not be used as abrasive particles gets embedded on the surface of the contact.

Arc chutes are checked and cleaned and also alignment is checked.

All internal wiring must be in good condition.

All connection to be checked for tightness.

All mechanical linkages to be checked for any sign of wear

The racking mechanism for removing breaker for servicing to localised position checked for free movement.

The protective devices like under voltage, low frequency and overload relays can be tested either by simulation or practically reducing values below the set values.

Circuit breaker, time relay mechanism with oil dashpots must have the pots filled to correct level, with clean oil of type recommended by manufactures.